A User Equipment (UE), also known as a mobile station, wireless terminal and or mobile terminal is enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between UEs, between a UE and a wire connected telephone and or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The UE may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The UE in the present context may be, for example, portable, pocket-storable, hand-held, computer comprised, or vehicle-mounted mobile devices, enabled to communicate voice and or data, via the radio access network, with another entity, such as another UE or a server.
The wireless communication network covers a geographical area which can be divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and or terminology used.
Sometimes, the expression “cell” may be used for denoting the radio network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on radio frequencies with any UE within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the UE. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node.
Beyond 3G mobile communication systems, such as e.g., 3GPP LTE, offer high data rate in the downlink by employing multiple antenna systems utilizing Multiple-Input and Multiple-Output (MIMO).
Large MIMO systems, also denoted high level MIMO systems or massive MIMO systems, are developed from the MIMO systems and use large antenna arrays to improve throughput of wireless communication systems.
Generally, in order to achieve a good performance, a relatively complex receiver design, such as a maximum-likelihood detector (MLD), is needed. There have been a number of attempts to reduce the complexity of the MLD, whereby a number of variants of near-MLD algorithms have been proposed, such as for example soft output sphere decoding (SOSD), QR-decomposition with M algorithm (QRD-M), K-Best algorithms etc. These near-MLDs are often capable of reducing the complexity in some respect. However, the lower complexities of these near-MLDs usually come at the price of scarifying the performance. Thus, in order to obtain a promising performance close to the MLD performance, the complexities of these conventional variants will also be close to the complexity of the MLD.
In order to achieve a high spectral efficiency in the LTE downlink, UE categories 5 to 8 will support 4×4 MIMO transmission and 8×8 MIMO transmission, as has been defined by the 3GPP standardization documents. The DL peak rate can be up to approximately 300 Mbps for UE category 4, and 3Gbps for UE category 8.
MLD works well for relatively small MIMO layer numbers, such as 2×2 MIMO or 3×3 MIMO. However, as the complexity of MLD increases exponentially with the number of MIMO layers and with the modulation type, it is prohibitive for a UE to utilize MLD for large number of MIMO layers, such as for 4×4 MIMO transmission and 8×8 MIMO transmission. For instance, for UEs that support 8×8 MIMO and use 64 Quadrature Amplitude Modulation (QAM) for all 8 layers, the MLD algorithm has to search through all 648 possible transmitted symbol combinations. Such an extensive search procedure of course results in an enormous and disadvantageous complexity for the MLD.
There are also other types of detectors available, such as the Linear Minimal Mean Square Error (LMMSE) receiver. The LMMSE receiver has a very low computational complexity that only increases polynomially with the number of MIMO layers. This polynomial complexity increase can be compared with MLD, which increases the complexity exponentially with the number of MIMO layers. However, as is widely-known in the field, the performance of the LMSSE receiver is sub-optimal compared with MLD, especially when the channel is highly spatially correlated.
Usually, parallel interference cancellation (PIC) or serial interference cancellation (SIC) techniques are used in the LMSSE receivers to improve the LMMSE performance. LMMSE receivers utilizing PIC or SIC can utilize the Log-Likelihood Ratio (LLR) feedbacks. In this document, the notation Log-Likelihood Ratio (LLR) is used for outputs from an outer Error Correction Code (ECC) decoder, or for outputs being iterated by the LLMSE and/or MLD detectors thereby excluding the outer ECC decoder. The self-iterated LMMSE with PIC or SIC has low complexity and less process latency than the LMMSE scheme involving the outer ECC decoder. However, the self-iterated LMMSE often suffers from big and unacceptable performance losses.
The complexity problems for the MLDs and the low performance of LMMSE receivers bring major challenges for category 5˜8 LTE UE detectors, which should keep the complexity low while maintaining a good performance under different channel conditions, modulation types and coding rates. In addition, with Channel Quality Indicator (CQI) feedback being utilized, the number of transmitted MIMO layers from the radio network node may vary over time, such that a first number of layers are used at a first point in time and a second number of layers are used at a second point in time. Therefore, the compatibility for the UEs to be able to support differing numbers of transmitted MIMO layers is an important aspect regarding size and/or production costs for the UE. Conventional detector solutions that do not support differing layer numbers adaptively thus had to be equipped with multiple detectors, e.g. one separate detector for each number of layers, to be able to provide compatibility to varying use of layer numbers. This adds to the UE size since it adds to the actual die size and the cost of the processor and or memory chipset.
Thus, conventional detectors suffer from high complexity and/or poor performance. This is explained more in detail below, with reference to a signal model used.